Rotor attachment assembly
BACKGROUND OF THE INVENTION 
     I. Field of the Invention 
     The present invention relates to rotor attachment assemblies, and more particularly relates to such a structure which provides for coaxiality of a rotor and a rotor shaft and which thereby provides against bending of the shaft. 
     II. Description of the Prior Art 
     A variety of conventional mechanisms include rotors or the rotating part of a mechanical device. Conventional turbine engines and turbomechanisms such as turbochargers as the primary examples include impellers as rotors which are driven by fluids. According to the prior art, customary practice has been to mount the impeller, which is that member of the rotor class of which we are primarily concerned, on a drive shaft. To rigidly fix the impeller (or rotor) to the drive shaft, a substantially standard nut has conventionally been used. Known drive shafts include a smooth body which terminates at a shoulder leading to the nut-receiving threads defined in the end portion of the shaft. 
     As is conventionally known, the length of the smooth body is equivalent to the length of the shaft-receiving aperture in the impeller. Accordingly, when the impeller is fully pressed upon the shaft, only the threaded portion of the shaft extends beyond the rotor. The rotor is then secured to the shaft by a conventional nut, possibly in combination with one or more flat or lock washers, which press against the rotor. The nut base (or a washer) and the surface of the rotor mate with one another in this construction. 
     Known methods of fastening a rotor to a rotor shaft suffer from a variety of disadvantages. First, using conventional retention methods, the shaft may bend in response to non-uniform loading due to improperly manufactured tolerances. These errors, even though relatively slight, nevertheless cause non-parallelism of the assembled rotor and the nut mating surfaces. 
     A bent rotor shaft results in at least two problems which affect performance. First, the rotor&#39;s center of gravity is shifted resulting in rotational imbalance. Second, the rotor embodies increased radial runout, thus reducing or possibly eliminating fin clearance. 
     In response to these undesirable characteristics, attempts have been made to minimize shaft bending by incorporating spherically-faced nuts and washers or by making manufacturing tolerances more stringent. However, spherically-faced nuts and washers require trial-and-error adjustment. Readjustment is often subsequently required. More stringent manufacturing tolerances result in manufacturing cost increases. 
     Accordingly, prior inventions have failed to eliminate the problems commonly associated with known rotor assembly designs. 
     SUMMARY OF THE PRESENT INVENTION 
     The present invention is a rotor attachment assembly for securing a rotor, such as a compressor impeller, to a rotor shaft. The assembly comprises a locking nut, a rotor, and a nut receiving rotor shaft. The shaft comprises a smooth body and a threaded end which is of less diameter than the shaft body. The nut includes an unthreaded bore portion and a threaded portion wherein the unthreaded bore is of greater diameter than the threaded portion, thus comprising a counterbore construction. The smooth body of the shaft and the unthreaded bore of the nut have a very close tolerance, so close as to preferably comprise an interference fit. The threaded end of the shaft and the threaded portion of the nut have a class two fit. 
     When the rotor of the present invention is in place on the shaft, part of the smooth body extends beyond the nut flush mounting surface of the rotor. The bore portion of the nut is deeper than the length of the part of the smooth body extending beyond the rotor. Accordingly, when the nut is tightened onto the shaft to press against the rotor, a gap is left between the body of the shaft and the threaded portion of the nut. 
     This construction offers a number of significant advantages over the known art. First, the screw-to-nut thread clearance is increasable to a class two (free fitting) thread. Second, by piloting the nut onto the shaft by means of the interference fit counterbore rather than by the threads alone, coaxiality of the shaft and nut are assured. Accordingly, the parallelism of the assembled rotor and the nut mating surfaces is dependent upon the perpendicularity of those surfaces to their bores. With proper manufacturing control of perpendicularity of those surfaces, shaft bending and its associated problems are eliminated in a low cost manner. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The present invention will be more fully understood by reference to the following detailed description of the preferred embodiment of the present invention when read in conjunction with the accompanying drawing, in which like reference characters refer to like parts throughout the views, and in which: 
     FIG. 1 is a perspective, partially exploded view illustrating the rotor attachment assembly according to the present invention; and 
     FIG. 2 is a cross-sectional view of an assembled rotor attachment assembly according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT OF THE PRESENT INVENTION 
     FIGS. 1 and 2 show a preferred embodiment of the present invention. While the configuration according to the illustrated embodiment is preferred, it is envisioned that alternate configurations of the present invention may be adopted without deviating from the invention as portrayed. The preferred embodiment is discussed hereafter. 
     With reference to FIG. 1, there is shown an exploded view of a rotor attachment assembly generally indicated as 10. The assembly 10 comprises a rotor 12, a shaft 14, and a nut 16. As illustrated, the rotor 12 is in place on the shaft 14 while the nut 16 has yet to be fastened to the shaft 14. 
     The rotor 12 iS illustrated as being a compressor impeller, although the present invention may be applicable to the rotating parts of many mechanical devices having similar configurations. The rotor 12 comprises a plurality of fins 18. 
     The rotor 12 also includes a nut mating surface 20 which is perpendicular to a rotor shaft receiving bore 22 (see FIG. 2) of the rotor 12. 
     The rotor shaft 14 includes a smooth body 24 (a portion of which is visible in this figure) and a threaded nut receiving portion 26. The nut receiving portion 26 is of less diameter than the smooth body portion 24, as illustrated. 
     With reference now to FIG. 2, a cross-sectional view of the rotor attachment assembly 10 is illustrated. According to this view, all of the elements of the assembly are in place in their assembled and tightened positions. 
     Also this view, the full length of the smooth body 24 of the shaft 14 is visible. A portion of the body 24 can be seen extending beyond the nut mating surface 20 of the rotor 12. The rotor 12 preferably has a rotating fit with respect to the smooth body 24 of the shaft 14. 
     The nut 16 has defined therein a threaded hole 28 which is substantially matable with the threaded nut receiving portion 26 of the shaft 14. As discussed above, the screw-to-nut thread clearance need only be a class two (free fitting) thread. 
     The nut 16 also has defined therein a shaft locating hole or counterbore 30 which is substantially matable with the portion of the smooth body 24 which extends beyond the nut mating surface 20 of the rotor 12. 
     The counterbore 30 and the body 24 should be machined to a close tolerance, preferably resulting in an interference fit. At the base of the nut 16 there is a rotor mating surface 32 which is perpendicular to the counterbore 30 and the threaded hole 28. 
     When the nut 16 is fully threaded onto the shaft 14, the nut mating surface 20 of the rotor 12 and the rotor mating surface 32 of the nut 16 mate with each other with a close parallel fit, as illustrated. With the nut 16 thus in its locking position as illustrated in FIG. 2, it can be seen that the counterbore 30 is deeper than the length of that portion of the body 24 extending beyond the nut mating surface 20 of the rotor 12. Thus even with the nut 16 fully in place, a gap remains between the body 24 and the base of the threaded hole 28 of the nut 16. According to this construction, as noted above, by piloting the nut 16 onto the shaft 14 by means of the interference fit counterbore rather than by threads alone, coaxiality of the shaft 14 and the nut 16 are assured.